Some patients suffer from deterioration of the lung function usually as a result of a chronic respiratory disease like bronchitis, emphysema and pulmonary fibrosis. When deterioration of the lung function occurs, the patient becomes hypoxemic. To treat hypoxemia and to relieve the ailments associated therewith, a health care provider will, most likely, prescribe supplemental oxygen to the patient so that the patient can inhale the supplemental oxygen along with ambient atmospheric air in order to maintain a sufficient oxygen concentration level in the blood stream.
Early supplemental oxygen delivery systems included a source of oxygen such as a tank of oxygen connected in fluid communication with a nasal cannula structure. Oxygen was delivered on a continuous flow basis, albeit a low, fixed flow rate, throughout the entire breathing cycle to the nose of the patient by a tube which interconnected the source of oxygen with the nasal cannula structure. Although efficacious in maintaining oxygen concentration levels in non-ambulatory patients, costly oxygen was lost to the ambient atmosphere since the continuous flow of oxygen was provided to the patient's nose during the entire breathing cycle, i.e., regardless if the patient was inhaling or exhaling. It was obvious that much of the oxygen that was being delivered to the nose of the patient was being wasted when using this continuous-flow supplemental oxygen delivery system. Furthermore, this early supplemental oxygen delivery system was unable to deliver variable quantities of oxygen in response to the changing oxygen demands of the patient when the patient's activity level changed. With the rising costs of medical care and the need to provide a more effective means of delivering oxygen to hypoxemic patients, other more effective oxygen delivery systems were developed.
To better comprehend the deficiencies of these prior art supplemental oxygen delivery systems and devices that conserved oxygen during operation, it would be beneficial to understand the breathing cycle of the patient before discussion of this prior art. When plotting the breathing pressure of the patient as a function of time, one breathing cycle generally appears as a modified sine wave. The positive breathing pressure of the sine wave as it rises then falls relative to ambient air pressure reflects the exhalation interval of the breathing cycle; correspondingly, the negative breathing pressure of the sine wave as it continues to fall after termination of the exhalation cycle then rises relative to the ambient air pressure reflects the inhalation interval of the breathing cycle. However, in reality, the sine wave of the breathing cycle is skewed whereby the exhalation interval of the skewed sine wave constitutes on an average of about two thirds of the breathing cycle while the inhalation interval of the skewed sine wave constitutes on an average of one third of the breathing cycle.
Furthermore, the respiratory system of the patient includes the passageway to the lungs comprising the nares of the nose, the nasal cavity and the trachea which together provide a conduit for transporting ambient atmospheric air to a person's lungs. This passageway is anatomically dead space that, after the exhalation interval, is now filled with exhaled air which, in turn, becomes the first quantity of inhaled air during the subsequent inhalation interval. By way of example only, on the average, this anatomically dead space retains about the first one third (1/3rd) of the quantity of air for the next inhalation. The remaining two thirds (2/3) of the quantity of air required for breathing is provided by fresh ambient atmospheric air during the subsequent inhalation interval. Only one half (1/2) of this fresh ambient air reaches the lungs for gaseous exchange, i.e., the second one third (1/3) of the required air (or the first one half (1/2) of the fresh air) is carried to the lungs and the last one third (1/3rd) of the required air (or the second one half (1/2) of the fresh air) remains in the anatomically dead space. Therefore, on the average, only 16% to 17% of the breathing cycle brings fresh air or fresh air combined wit insufflation gas to the lungs and this occurs only during the first one half (1/2) of the inhalation interval of the breathing cycle.
In response to the waste of oxygen associated with the early prior art supplemental oxygen delivery systems that provided a continuous flow of oxygen throughout the entire breathing cycle, many other prior art systems and devices have been developed and implemented for delivery of supplemental oxygen to patients which included oxygen-conserving features. Some of these devices were characterized as being capable of providing oxygen "on demand" to the patient or "on the go". Generally, "on demand" meant in these systems that oxygen was not delivered to the patient until after the beginning of the inhalation interval of the breathing cycle and that no oxygen was delivered to the patient during any portion of the exhalation interval of the breathing cycle. Since oxygen was not delivered to the patient during the exhalation interval which constitutes two thirds of the entire breathing cycle, significant quantities of oxygen were conserved. Two types of the "on demand" supplemental oxygen delivery systems are discussed immediately below.
U.S. Pat. No. 4,462,398 and U.S. Pat. No. 4,519,387 to Durkan et al. reveal respirating gas supply methods and apparatuses designed to conserve the respirating gas during patient insufflation. A control circuit responsive to a sensor operates a valve to supply pulses of respirating gas through a single hose cannula to a respiratory system of a patient when negative pressure indicative of the initial stage of inspiration is sensed by the sensor. The pulse of gas delivered to the respiratory system can have a preselected pulse profile. This method provides for supplying a fixed volume of supplemental respiratory gas per unit of time. The volumetric flow rate of the supplemental respiratory gas is preset and the time duration of each application of the supplemental respiratory gas is preselected, thereby providing a fixed volume of respiratory gas after the beginning of inhalation. Also, this method provides for a minimal delay interval between successive applications of respiratory gas and such delay interval is also predetermined since the time interval for respiratory gas flow is preset for a time less than the time of the inspiration.
Another prior art supplemental oxygen delivery system designed to conserve respiratory gas by delivering oxygen "on demand" only during inhalation is described in U.S. Pat. No. 4,612,928 to Tiep et al. which discloses both a method and apparatus for supplying a gas to a body. The apparatus and method are employed to minimize the amount of oxygen needed to maintain a specific oxygen concentration level in the blood of an individual. The apparatus includes a transducer and other circuit components to obtain a first series of pulses or signals corresponding to the individual's breath rate. A divider or counter processes the signals or pulses of the first series to create a second series of pulses or signals corresponding to periodic pulses or signals of the first series. The pulses or signals of the second series are used to periodically open a valve to deliver oxygen to the individual at about the start of the inhalation interval of the individual's periodic breathing cycles.
In U.S. Pat. No. 4,457,303 and U.S. Pat. No. 4,484,578, Durkan recognizes that oxygen delivered at the end of the inhalation interval of the breathing cycle is wasteful. These two patents describe respirator apparatuses and methods therefor. In brief, a fluidically-operated respirator comprises an apneic event circuit and a demand gas circuit. The apneic event circuit comprises a variable capacitance device and an exhaust means which rapidly discharges fluid from the circuit when inhalation occurs. The demand gas circuit of the respirator supplies respirating gas to a patient at the beginning of inhalation and for a time period which is a fraction of the duration of the inhalation. Thus, these patents also follow the reasoning that insufflation at the beginning of inhalation will effectively supply the respirating gas to the patient.
One prior art supplemental oxygen delivery system begins to deliver a steady flow of oxygen during a later stage of the exhalation interval and through an advanced stage of the inhalation interval of the breathing cycle and superimposes upon this steady flow of oxygen a peak pulse flow of oxygen at the beginning of inhalation. This is described in U.S. Pat. No. 4,686,974 to Sato et al. which discloses a breath-synchronized gas-insufflation device. This device includes a gas source, a valve, an insufflating device, a sensor, and an operational controller. The valve is connected to the gas source so as to regulate flow rate and duration of the gas flow from the gas source. The insufflating device is connected to the valve so as to insufflate the gas therefrom toward a respiratory system of a living body. The sensor is exposed to respiration of the living body and produces electric signals which must distinctively indicate an inhalation interval and an exhalation interval of the breathing cycle. The operational controller receives the electric signals from the sensor and produces control signals to the valve so that gas insufflation starts before the beginning of the inhalation interval and ends before termination of the inhalation interval while providing a short pulse-like peak flow of a large amount of the gas in an early stage of the inspiratory interval. Specifically, steady insufflation of the gas starts before the beginning of each inhalation and the pulse-like peak flow insufflation of the gas is superimposed on the steady insufflation for a short period of time after the beginning of the inhalation. An arbitrary time interval based upon an average exhalation period and an average inhalation period is chosen to trigger and end insufflation during the breathing cycle.
Although the prior art devices discussed hereinabove indeed conserved oxygen, they failed to address the problem related to the changing respiratory needs of the patient that vary with different patient activity levels. When a patient requiring supplemental oxygen is at rest, relatively small quantities of oxygen are needed to maintain appropriate levels of oxygen concentration in the blood and thereby prevent what is termed "desaturation". With an increase in the physical activity of a patient, larger quantities of oxygen are needed to maintain appropriate levels of oxygen concentration in the blood compared to when the patient is at rest.
In U.S. Pat. No. 4,706,664, Snook et al. disclose a pulse-flow supplemental oxygen apparatus which yields savings in oxygen while affording the patient the physiological equivalent of a prescribed continuous stream of oxygen. The apparatus includes a demand oxygen valve operated in a pulse mode by means of electronic control circuitry. Through an appropriate sensor, the electronic control circuitry monitors the patient's breathing efforts and gives a variable timed pulse of oxygen to increase the volume delivered to the patient during the very initial stage of each inhalation interval of the breathing cycle or breaths. Pulse volume variability is based upon a measured parameter characterizing a plurality of the patient's preceding breathing cycles. The elapsed time interval of the patient's three preceding breathing cycles is measured to effectively measure the rate of the breathing cycles. These breath-characterizing parameters, together with data characterizing the prescribed continuous oxygen flow to be matched, enable the apparatus to give the desired dose-variability.
U.S. Pat. No. 4,584,996 to Blum reveals a method and apparatus for intermittent administration of supplemental oxygen to patients with chronic lung disfunction. The apparatus is programmable for administering the specific oxygen requirements of the patient and is responsive to changes in these oxygen requirements with increased patient activity. The patient's arterial blood oxygen level is measured while supplying oxygen to the patient during inspiration to determine the number of breathing cycles required to reach a first higher arterial blood oxygen level and is again measured without supplemental oxygen to determine the number of breathing cycles required to diminish the arterial blood oxygen level to a second, lower level. These two cycle numbers are utilized in an algorithm which is applied as a program to the apparatus having a breathing cycle sensor, a counter and control valve. The control valve provides a regulated flow of supplemental oxygen to a nasal cannula for a predetermined number of "ON" breathing cycles and to shut off the flow for a preset number of "OFF" breathing cycles sequentially and repetitively, thereby conserving oxygen while medically monitoring the patient's blood oxygen levels. The oxygen conservation features of this apparatus are further enhanced by turning off the oxygen flow during the exhalation interval of each breathing cycle throughout the "ON" breathing cycles. As the respiratory rate of the patient increases with patient activity, the duration of the "ON" and "OFF" periods changes accordingly.
In U.S. Pat. No. 4,686,975, Naimon et al. teaches a supplemental respiratory device that uses electronic components to intermittently regulate the flow of a respirable gas to a user on a demand basis. By monitoring small changes in the relative airway pressure, this respiratory device supplies gas only when an inhalation is detected. This respiratory device can also vary the duration of the gas delivery time to compensate for changes in the user's breath rate, thereby attempting to adjust for changes in the patient's respiratory needs based upon activity.
Presently, many manufacturers are marketing oxygen conserver devices which are adapted to retrofit onto typical supplemental oxygen delivery systems that employ any type of oxygen source such as portable oxygen tanks, oxygen concentrators or wall outlet supplies often utilized in hospitals. These oxygen conserver devices are adapted to be interposed between the oxygen source and a conventional nasal cannula structure. Medisonic U.S.A., Inc. of Clarence, N.Y., manufactures an oxygen conserver device entitled MEDISO.sub.2 NIC Conserver. It conserves oxygen by interrupting the flow of oxygen from the source to the patient during the exhalation interval of the patient's breathing cycle. Chad Therapeutics, Inc. of Chatsworth, Calif., manufacturers an oxygen conserver device bearing a registered trademark, OXYMATIC.RTM. Electronic Oxygen Conserver. Chad's oxygen conserver eliminates oxygen waste during both the exhalation interval and the later portion of the inhalation interval of the breathing cycle. TriTec, Inc. of Columbia, Md., manufactured a demand oxygen cannula for portable oxygen systems that also responded to the negative pressure of inhalation. Smith-Perry Corporation of Surrey, British Columbia, Canada, manufactures The VIC (Voyager Intermittent Controller) Breathsaver that senses every breath of the patient and delivers a measured dose of oxygen only when the patient inhales. Pulsair, Inc. of Fort Pierce, Fla., manufactures an oxygen management system that delivers oxygen to the patient "on demand" at the initiation of inhalation. The Henry G. Dietz Co., Inc. of Long Island City, N.Y., manufactures an oxygen conserver device entitled HALA'TUS 1 which conserves oxygen by sensing when inhalation takes places and delivers the oxygen only during inhalation.
None of these oxygen conserver devices deliver oxygen to the patient during any stage of exhalation.
Although many improvements have been made to conserve oxygen while employing supplemental oxygen delivery systems, there remains a need in the industry to more efficiently and effectively deliver sufficient concentrations of oxygen to a patient under changing conditions of physical activity while simultaneously conserving oxygen. There is a need to provide an intermittent gas-insufflation apparatus that can supply the appropriate quantity of oxygen to be delivered to the patient during an exhalation interval of an immediate breathing cycle and into a subsequent inhalation interval of a successive breathing cycle. It would be advantageous if delivery of the appropriate quantity and concentration of oxygen commences during the exhalation interval of the immediate breathing cycle. It would also be advantageous if this intermittent gas-insufflation apparatus could deliver the appropriate quantity and concentration of oxygen during the exhalation interval of the immediate breathing cycle and into the subsequent inhalation interval of the successive breathing cycle. There is also a need for an intermittent gas-insufflation apparatus that can determine an appropriate flow rate profile during the immediate breathing cycle. This flow rate profile would be designed to deliver the appropriate quantity and concentration of oxygen to the patient commencing during the exhalation interval of the immediate breathing cycle and into the subsequent breathing cycle. It would be advantageous if the intermittent gas-insufflation apparatus delivers a portion of the appropriate quantity of oxygen at a nominal flow rate to the patient during the exhalation interval of the immediate breathing cycle so that a portion of the residual air found in the nasal cavity from a prior breathing cycle can be purged therefrom and a remaining portion of this residual air becomes enriched with oxygen in preparation for the subsequent inhalation interval of the successive breathing cycle. Also, there is a need for an intermittent gas-insufflation apparatus that terminates oxygen insufflation during the subsequent inhalation interval of the successive breathing cycle. It would be advantageous if the intermittent gas-insufflation apparatus would cease to deliver the appropriate quantity of oxygen during the subsequent inhalation interval of the successive breathing cycle before a negative peak pressure value determined in the immediate breathing cycle is reached in the successive breathing cycle. The present invention satisfies these needs and provides these advantages.